This invention relates generally to communication systems and more particularly to oscillators therefor.
Oscillators are of fundamental importance in communication systems. These systems have exacting requirements for oscillators. These requirements include: spectral purity, including short term frequency stability, often expressed in terms of phase noise, and harmonic distortion (deviation from ideal waveform); ease of frequency control (including, in some instances, wide tuning range); and low power consumption. In addition, ease of integration in monolithic form is often an additional requirement since most commercial communication systems must now be manufacturable at a low cost.
Low phase noise is especially critical in communication systems. For example, consider an oscillator operating at 1 GHz driving a mixer in a narrow-band FM radio application. If the short-term frequency varies in a random fashion, with a normal standard deviation of only 10 parts per million, the resulting base band signal will have a noisy frequency modulation of 10 kHz RMS, which may be two to three times the variation due to the signal modulation, making it useless. Accordingly, the phase noise of a high frequency oscillator in a communication system must be extremely low.
One class of oscillators that has been used in communication systems are those depending on time-constants formed by capacitors (C) and resistors (R). Within this category are so-called relaxation oscillators (multivibrators), which use at least one capacitor, and filter-based CR oscillators, which require at least two capacitors. This class of oscillators, along with all other oscillators, achieve an oscillatory condition by connecting two nominally ideal integrator stages in a loop including a sign change. A model of this type of oscillator is shown in FIG. 1.
The model in FIG. 1 includes a non-inverting integrator followed by an inverting integrator with the output of the inverting integrator connected to the input of the non-inverting integrator to form a closed loop. Because the non-inverting integrator introduces a constant phase lag of 90 degrees, the output signal of the non-inverting integrator V(q) has a constant phase lag of 90 degrees relative to the input signal V(i). As such, the output signal is said to be "in quadrature" with the input signal. Accordingly, oscillators that can be modeled as shown in FIG. 1 are called quadrature oscillators.
A popular implementation of the oscillator modeled in FIG. 1 is shown in FIG. 2. The oscillator of FIG. 2 includes two op-amps A1 and A2, each configured as an integrator by having a capacitor coupled between the input and the output of the associated op-amp. The oscillator of FIG. 2 also includes two multiplier circuits M1 and M2 that are used to tune the oscillator to a desired frequency of oscillation. The multipliers effectively vary the time constant associated with each integrator stage responsive to a frequency control signal applied thereto. Implementation shown in FIG. 2 is referred to as a voltage-controllable state-variable oscillator.
The voltage controllable state-variable oscillator of FIG. 2 is fundamentally unsatisfactory for high frequency, low phase noise applications. The primary reason for this is that the op-amp itself is fundamentally an integrator, even without the addition of the capacitor. Indeed, op-amps are consciously designed to be integrators, though generally not suited for direct use as such. An internal resistor (which sets the gm of the op-amp input stage) and an internal capacitor form a dominant pole in the op-amp which introduces an additional 90 degrees of phase lag in the integrator circuit at high frequencies. In addition, the dominant pole produces a change in the amplitude slope of the integrator. This dominant pole limits the state-variable oscillator to quite low frequencies. Moreover, the finite bandwidth of the multipliers imposes an additional limitation on the upper end of the frequency range which can be attained by the voltage-controllable state-variable oscillator.
Accordingly, a need remains for a high-frequency oscillator having improved phase noise.